A spark plug is used as an ignition means of internal combustion engines such as engines for automobiles. In general, faces of center and ground electrodes, which face each other, are respectively disposed with electrode tips to improve ignitability of the spark plug. The electrode tips, for example, are made up of noble metal materials and have columnar shapes. A predetermined spark discharge gap is formed between the electrode tip of the center electrode, which extends to the ground electrode, and the electrode tip of the ground electrode. In addition, spark discharge is generated between the electrode tips which face each other, and the spark discharge ignites an air-fuel mixture.
In addition, changing configurations of the center and the ground electrodes enables reduction in usage of the noble metal materials. In one instance, Japanese patent No. 4775447 will be referred to as patent document 1. Patent document 1 discloses that a welded part is formed on at least a part of a tip end face of a convex part. The welded part is formed by welded the noble metals with a part of an electrode base material. The convex part is a part of a base material of the ground electrode and is projected to the center electrode faced to the ground electrode. Furthermore, in patent document 1, it is proposed that a covering layer including the noble metals is formed on a corner part and a part of a side face of the convex part, thereby enable to suppress a consumption of the corner part and the part of the side face of the ground electrode.
In addition, a rapidly mixed gas stream is formed in a combustion chamber to improve combustion quality in the internal combustion engine. This method of using the rapidly mixed gas stream enables an initial flame to be a large flame by inducing the spark discharge to a center of the combustion chamber. However, the spark discharge may flow to a lateral direction of the ground electrode and reach a base of the convex part of the ground electrode. In this case, the spark discharge reaches a side face of the ground electrode which is not coated with the noble metals, and a problem may occur that the ground electrode is easily consumed. In addition, oxidation of the noble metal materials progresses in a high-temperature atmosphere or if heat stress is repeatedly applied to the ground electrode. Thereby, the noble metal coating layer may detach from the electrode base material.